In situations involving complex structures and steeply dipping reflectors often found in complicated subsurface geological formations such as those in and below salt flanks, current ray-based migration methods have significant limitations due to waveform multi-pathing, including caustic and prismatic waves. Reverse time migration (RTM), however, can better handle those complicated wavepaths by correlating wave fields that are properly propagated forward in time for sources and backward in time for receivers. Two drawbacks of RTM when applied to a surface seismic profile (SSP) are: (1) imaging artifacts from backscattering; and (2) unbalanced amplitudes. Those undesirable effects can be removed or reduced by applying: (1) a Laplacian filter; (2) an imaging condition; and (3) an obliquity compensation factor.
For a vertical seismic profile (VSP), RTM imaging artifacts may also arise from limited aperture and data coverage. Such artifacts cannot be eliminated using the above-mentioned filters since one cannot compute such filters from VSP data alone. To illustrate, FIG. 1A shows a 2-D VSP survey and corresponding survey geometry, and FIG. 1B shows an RTM image computed using synthetic data that was generated using the survey geometry of FIG. 1A. In FIG. 1A, the “X” at the surface (zero depth) represents the source position and the receivers are distributed directly below in a vertical borehole from 1200 meters to 2700 meters. The heavy bars on the layer (acoustic impedance) boundaries indicate the portions of those boundaries for which data can be collected for the given source/receivers configuration. In FIG. 1B, the image area bounded by the superposed quadrilateral represents the portion of the subsurface formation that the VSP data can cover. Thus, all the plot features outside that region are considered imaging artifacts that should be suppressed. In addition, even within the targeted (i.e., interior) region, there are other artifacts (e.g., “swing” artifacts) that do not correspond to real spatial reflectors.